1. Field of the Invention
The present invention relates to an aluminum-alloy rolled sheet for forming and a production process therefor. More particularly, the present invention relates to an aluminum-alloy rolled sheet for forming, which is suitable for applications in which a high strength is required and which has been subjected to paint baking, such as in an application for an automobile body, as well as to a process for producing the same.
2. Description of the Related Arts
In many cases, cold-rolled steel sheets are used for an automobile body, but because of recent demands for a lowering of the weight of the automobile body, consideration is being made of the use of an aluminum-alloy rolled sheet for that purpose. Since the sheet for an automobile body is formed by pressing, the requirements therefor include an excellent formability, particularly in stretching and bulging, non-generation of Leuders' lines during the forming operation, and a high strength, particularly after the sheet is subjected to paint baking.
Various aluminum-alloy sheets have been used for formed products, and the main alloys are classified by their alloying component series as follows;
A. A non-heat treatable type Al-Mg alloy, such as 5052 alloy in the O temper (2.2.about.2.8% Mg, 0.15.about.0.35% Cr, the being balance Al and unavoidable impurities) or 5182 alloy in the O temper (0.20.about.0.50% Mn, 4.0.about.5.0% Mg, the balance being Al and unavoidable impurities).
B. A heat treatable Al-Cu series alloy such as a 2036 alloy in T4 temper (2.2.about.3.0% Cu, 0.1.about.0.4% Mn, 0.3.about.0.6% Mg, and the balance being Al and unavoidable impurities).
C. A heat treatable Al-Mg-Zn-Cu series alloy in T4 temper. Note, the alloys of these series are disclosed in Japanese Unexamined Patent Publication Nos. 52-141,409, 53-103,914, and 57-98,648.
D. A heat treatable Al-Mg-Si series alloy, e.g., 6009 alloy in T4 temper (0.4.about.0.8% Mg, 0.6.about.1.0% Si, 0.15.about.0.6% Cu, 0.2.about.0.8% Mn, the balance being Al and unavoidable impurities) in T4 temper or 6010 alloy (0.6.about.1.0% of Mg, 0.8.about.1.2% Si, 0.15.about.0.60% Cu, 0.2.about.0.8% Mn, the balance being Al and unavoidable impurities).
Nevertheless, it is difficult to completely satisfy all of the above described requirements by the above mentioned conventional aluminum alloys.
That is, the strength of the alloy "A" is not satisfactory, in that wrought products of this alloy have problems wherein Leuders' marks are liable to occur during the forming process and, further, that the strength is lowered during the paint baking process.
The alloy "B" has the problems of a poor formability and a strength reduction during the paint baking process. The alloy "C" does not have a satisfactory formability, particularly a bending property. This alloy also has the problem of a strength reduction during the paint baking process. The alloys "D" have an unsatisfactory strength for the 6009 alloy and unsatisfactory stretching and bending characteristics for the 6010 alloy.
As described above, with the conventional aluminum alloys, it is difficult to completely satisfy the requirements for a sheet to be used for an automobile body, i.e., an excellent formability, particularly the stretching and bulging formability, no generation of Leuders' marks, and a high strength, particularly the strength after paint baking. To solve these problems, in Japanese Unexamined Patent Publication Nos. 61-201748 and 61-201749 the present inventors proposed an aluminum-alloy rolled sheet for forming having an improved balance in strength and formability and generating no Leuders' marks, as well as a production method for the same.
The present inventors carried out further studies of an aluminum-alloy sheet for forming, and arrived at the following conclusions. The rigidity of an automobile body is virtually controlled by the modulus of elasticity of the used material. Therefore, when an aluminum-alloy material having a lower modulus of elasticity than that of a cold-rolled steel sheet is used, a limitation arises in that the sheet thickness cannot be reduced, even if the material's strength has been enhanced. Under these circumstances, provided that a yield strength of 15 kg/mm.sup.2 or more is ensured after the paint baking, a further enhancement of strength is less advantageous than a further enhancement of formability, when applying an aluminum-alloy sheet to forming an intricate design of an automobile body. In other words, even if the strength as in the above two patent applications can be ensured only to a certain level, an intricate design of automobile body can be met by a further enhancement of the formability rather than the strength. The fields in which an aluminum-alloy sheet can be applied are thus broadened. Evidently, also in this case, Leuders' marks must not be generated during the forming.